The role of actin and myosin in the mechanism of cytokinesis will be studied, using sea urchin eggs as experimental material. I will attempt to integrate at both the cellular and molecular level information on the assembly, organization, and mechanism and control of comtraction of the contractile ring during cytokinesis. Using both chemical fixation and rapid freezing techniques, I will investigate the organization of actin filaments in the fertilized egg cortex and the relationship of this organization to the mechanism of contractile ring formation. The effects of pH and Ca ions concentration on the polymerization of cortical actin will be investigated and the role of actin polymerization in the mechanism of microvillar elongation will be studied. Actin and myosin - containing gels, derived from extracts of unfertilized sea urchin eggs, will be used a model system in which to study the regulation of actin-myosin interactions. Actin binding proteins will be isolated from gelled extracts and characterized. Antibodies will be prepared against purified actin binding proteins and used for the immunofluorescent localization of these proteins in the egg at different times in the cell cycle. Antibodies will also be micro-injected into living eggs to determine if these proteins function in the elongation of microvilli and in the formation of the cleavage furrow. Myosin will also be isolated from gelled extracts and characterized, and the possibility of myosin phosphorylation by a protein kinase in the egg cortex will be investigated. Finally, the mechanism by which actin is maintained in a non-filamentous state in the unfertilized cortex will be investigated.